1. Field of the Invention
The present invention generally relates to depositing molten solder onto a printed circuit board (PCB) or other electronic device. More particularly, a molten solder dispenser is provided which accurately deposits an extremely small amount onto the circuit board or device to accommodate the extremely close spacing present on integrated circuit devices (IC) which are currently being fabricated.
2. Description of Related Art
With advances in chip fabrication technology, an ever increasing number of integrated circuits are being built into the chips. This growing number of devices, requires a correspondingly greater number of input/output (I/O) connection points to be provided to the chips. The effect, to the computer fabrication industry, of closely spacing these I/Os is to require a system that can mechanically dispose and electrically connect the chips to a printed circuit board, for use in a computer system. Of course, it is well known to use solder as a means of providing both of these functions and there are many conventional systems for dispensing solder onto a printed circuit board.
For example, U.S. Pat. No. 4,828,886 uses a piezoelectric transducer to propel solder droplets onto a work piece. Other conventional systems (U.S. Pat. Nos. 3,222,776 and 4,754,900) utilize ultrasonic generators to create ultrasonic vibrations near a nozzle, or the like to urge solder onto a wettable surface.
Additionally, electromagnetic devices are known in the art which control the flow of metal through a conduit, or the like. For example, U.S. Pat. No. 4,842,170, describes an electromagnetic pump, which includes and alternating current coil adjacent a nozzle. The coil produces a magnetic field which induces a current in the liquid metal and urges the metal through the nozzle. The nozzle includes a nonconductive insert which redirects the induced eddy currents in the axial direction to enhance the metal flow. The insert may be positioned so as to impede the flow of the liquid metal such that the electromagnetic pump acts as a valve. U.S. Pat. No. 3,807,903 is another example of a conventional system wherein the magnetic field is varied to induce electrical current in the conductive metal flowing through a pipe. Additionally, varying the induced current changes the value of heat dissipation energy taken from the flow of the conductive liquid to hydrodynamically control the flow rate of the liquid.
U.S. Pat. Nos. 4,398,589; 4,566,859; and 3,515,898 all describe various electromagnetic pumps for conveying conductive material through a conduit. An "electromagnetic conduit" is described by U.S. Pat. No. 4,216,800 wherein electrical conductors are disposed along an axis which coincides with the axis to be imposed on a stream of conductive liquid. High frequency alternating currents are passed along successive conductors in opposite directions to contain the liquid stream.
Other conventional systems include those using ink jet technology to deposit droplets of solder on a PCB, or the like. In these systems, a piezoelectric crystal is used to urge the solder through a charge electrode and past deflection plates to guide the solder onto the substrate. It is noted that, piezoelectric materials exhibit poor temperature characteristics when used with molten metal materials, i.e. they are not capable of withstanding the elevated temperatures required for maintaining the material in a molten state.
In order to deposit the small amount of solder on a PCB that is now being required by today's technology, it is necessary to have a means that will not only urge to solder outward onto the board, but also reverse the outward flow of the solder and create and inward flow to effectively "snap off" the desired amount of solder. The previously discussed references which discuss the deposition of solder, use either ultrasonic energy or expansion and contraction induced by piezoelectric materials to urge the solder onto the board. While the piezoelectric materials are capable of pushing solder outward, each expansion of the material is coupled with a corresponding contraction. Therefore, the piezoelectric material is not capable of providing a constant pressure on the solder material, in either direction, as provided by the present invention. The ultrasonic energy effectively induces a high frequency vibratory motion that "shakes" the solder out of a nozzle. Further, both of these methods require electrical energy to be converted into another form (sound or mechanical) before any force is exerted on the solder. The previously described electromagnetic pumps also require that electrical energy be converted to magnetic energy before any force can act upon the conductive liquid. In order to reverse the direction of flow in these pumps, electric current in a first direction must be applied to a coil to set up the magnetic field. The direction of the current must then be reversed and the magnetic field collapses as the current passes through zero. The magnetic field is then setup in the opposite direction to induce current in the conductive material. All of these steps must occur before the force acting on the solder can be reversed. Those skilled in the art will understand that a relatively long amount of time is required for these steps to be implemented.
Therefore, it can be seen that a need exists for a system with the appropriate temperature characteristics that can apply a constant outward force of varying duration to a stream of solder and then substantially instantaneously reverse the direction of the force applied to the solder in order to "snap off" a droplet of solder for use in attaching chips having I/Os with extremely close spacing to a printed circuit board or device. The reverse direction of the force will also be constant and of a duration required by the application.